Organic Chemistry

8

a.

b.

7654 3210

87654 3210

CHCH 2 CH 3

CH 3

OH

CHCH 2 CH 3

CH 3

OH

e

c a

b

d

δ (ppm)

frequency

δ (ppm)

frequency

Section 14.17 Resolution of 1 HNMR Spectra 561

The sample used to obtain an NMR spectrum is made by dissolving the com-
pound in an appropriate solvent. Solvents with protons cannot be used since the sig-
nals for solvent protons would be very intense because there is more solvent than
compound in a solution. Therefore, deuterated solvents such as and are
commonly used in NMR spectroscopy.

14.17 Resolution of NMR Spectra

The NMR spectrum of 2-sec-butylphenol taken on a 60-MHz NMR spectrometer
is shown in Figure 14.30(a), and the NMR spectrum of the same compound taken
on a 300-MHz instrument is shown in Figure 14.30(b). Why is the resolution (separa-
tion of the signals) of the second spectrum so much better?

1 H

1 H

(^1) H
CDCl 3 D 2 O
1 H
Figure 14.30
(a) 60-MHz NMR spectrum of 2-sec-butylphenol. (b) 300-MHz NMR spectrum of
2-sec-butylphenol.
1 H 1 H
To observe separate signals with “clean”splitting patterns, the difference in the
chemical shifts of two adjacent protons ( in Hz) must be at least 10 times the value
of the coupling constant (J). The signals in Figure 14.31 show that as decreases,
the two signals appear closer to each other and the outer peaks of the signals become
less intense while the inner peaks become more intense. The quartet and triplet of the
ethyl group are clearly observed only when ¢n>Jis greater than 10.
¢n>J
¢n